Support token sinks other than end events

[nikku]

Is your feature request related to a problem? Please describe

BPMN has the concept of token sinks (consumers) that goes beyond end-events: In fact, every task without outgoing edges is a "sink", consumes a token, potentially closing the parent scope.

Right now we do not properly implement this, leading to 0 tokens being shown and execution being stuck (because the parent scope does not complete):

Describe the solution you'd like

• Scope completion for non-end-events is implemented in a BPMN 2.0 compliant manner

Describe alternatives you've considered

• Accept these diagrams as being non-modeling guideline conform "edge cases"